Many automotive vehicle of current manufacture come equipped with antilock braking systems and some with traction control systems as well. In a vehicle so equipped, the systems monitor the rotation of some, if not all, of the wheels—and certainly, the front wheels which steer the vehicle. Should a wheel begin to slip when the brakes are applied, as could well occur if the wheel encounters snow or ice, the antilock braking system will detect the loss of velocity and relax the braking forces on that wheel. This allows the wheel to continue to rotate and enables the driver to maintain better control over the vehicle. On the other hand, if one of the driving wheels encounters slippery pavement and as a consequence loses traction, the traction control system will apply a braking force to that wheel, and this has the effect of transferring the torque to the opposite wheel which perhaps has better traction.
An antilock braking system or traction control system for a vehicle thus requires speed sensors to monitor the rotation of some, if not all, wheels on the vehicle. While a variety of locations exist on a vehicle for installation of a speed sensor for a wheel, perhaps the best is in the housing that contains the bearing on which the wheel, or more accurately, the hub for the wheel, rotates. This keeps much of the sensor isolated from contaminates and objects that might otherwise damage it or disrupt its operation. In this regard, the typical sensor of this type fits into a cylindrical hole in the housing and has a stationary probe which is presented toward a target wheel or “tone ring” that rotates with the road wheel and contains discontinuities, such as teeth, which the sensor detects as the target wheel revolves. The result is a pulsating signal which reflects the angular velocity of the wheel. A control system monitors the signals from the wheel and initiates braking to achieve the results desired. These types of rotation sensors, commonly referred to as variable reluctance sensors, are found in a variety of applications, including gearboxes, drive shafts, and motors.
Typically, the due to the close proximity to the rotating or moving components required for proper functioning of a variable reluctance sensor, the sensor is exposed to the ambient temperature of the components. It is well known that the ambient temperatures of the components, such as within a vehicle wheel bearing housing or transmission are a good representation of the condition of the components. For example, a loss of lubricant or other cause of high friction, such as a contaminate or bearing failure, will result in an increase in component temperatures. In a vehicle wheel bearing application, an increase in wheel bearing temperatures may be caused by a significant application of vehicle brakes, either in a short and hard panic stop, or in a long and continuous fashion, such as regulating a vehicle's speed when descending a steep grade. Monitoring the temperatures of the components, such as the vehicle wheel bearings, provides useful information to a driver, and to a vehicle ECU, permitting early detection of possible damage, providing warnings, or permitting corrective actions to be taken.
Conventional systems to measure vehicle wheel bearing temperature utilize a separate temperature sensor placed in operative relationship to the vehicle wheel bearing, and which are coupled to the vehicle ECU or temperature warning components. In addition to requiring a separate sensor disposed in the vehicle wheel bearing or housing, this classical solution requires a separate electrical circuit for the temperature sensor signals, resulting in an increase in cost and additional susceptibility to wear or damage.
Systems which utilize properties of the antilock braking system or traction control system to provide a signal representative of vehicle wheel bearing temperature, such as shown in U.S. Pat. No. 5,381,090 to Adler et al. are known. In the '090 Adler et al. patent, the changes in direct current electrical resistance of the speed sensor windings responsive to the temperature of the associated wheel bearing and components are measured by applying a voltage to the speed sensor windings. The resulting output signal is a combination of the magnetically induced AC voltage representative of the vehicle wheel speed, and a DC voltage representative of temperature. Additional circuit components, including particularly active components are provided to filter the resulting output signal and provide a modified output signal in the form of a square wave output having a frequency related to vehicle wheel speed, together with a separate DC voltage for enabling temperature sensing.
It would be advantageous to provide a temperature sensing system for use in conjunction with existing variable reluctance sensors, such as those found in vehicle wheel bearings. A temperature sensing system utilized with existing vehicle antilock braking system (ABS) or traction control (TC) system passive variable reluctance sensors, which does not modify the magnetically induced AC voltage output signals which are representative of the vehicle wheel speed, would permit a bearing temperature sensing system to be retrofitted and installed inside the vehicle wheel bearings of vehicles without significant alteration to the vehicle wiring harness and electrical components. It would be particularly advantageous for applications where the ABS passive variable reluctance sensor is located inside the vehicle wheel bearing, as the temperature of the passive variable reluctance sensor correlates with the bearing internal temperature, permitting accurate monitoring of the bearing temperature.